A dynode, such as one disclosed in Japanese Laid-Open Patent Application No. S60-182642, in Japanese Laid-Open Patent Application No. H5-182631, or in Japanese Laid-Open Patent Application No. H6-314551, is known as this type of dynode. The dynode disclosed in Japanese Laid-Open Patent Application No. S60-182642 is a perforated plate member having a plurality of inwardly curved through-holes (e.g., barrel-shaped through-holes), and each of the through-holes is symmetric about its vertical axis and about a median plane passing through the dynode. The input and output diameters of the through-holes are the same, and are smaller than the diameter of the inside of the through-holes. The dynode consists of two metal sheets, and is structured such that the sheets formed by etching are disposed back to back with each other so as to allow openings larger in diameter of the convergent or tapered hole to face each other.
The dynode disclosed in Japanese Laid-Open Patent Application No. H5-182631 and Japanese Laid-Open Patent Application No. H6-314551 includes a plate having a plurality of through-holes one end of each of which serves as an input opening and the other end of each of which serves as an output opening, and an inner surface of each of the through-holes has an inclined part that inclines with respect to the incident direction of an electron so that the incident electron from an incident opening collides therewith. The output opening of each through-hole is formed to have a bore diameter larger than the input opening.
Meanwhile, a secondary electron emitted from an nth-stage dynode (“th” is a suffix used to form ordinal numbers) is guided by a control electric field formed by a potential difference between the nth stage and the (n+1)th stage, and is caused to impinge on the (n+1)th-stage dynode. In the dynode disclosed in Japanese Laid-Open Patent Application No. S60-182642, the input diameter and the output diameter of the through-hole are the same, and therefore an equipotential line cannot sufficiently enter the inside of the through-hole of the nth stage that functions as a control electric field, and, disadvantageously, the control electric field inside the through-hole is weak. Therefore, there is a case in which the emitted secondary electron returns to the side of the nth stage, this forming one cause by which the efficiency of gathering electrons is lowered.
In contrast, in the dynode disclosed in Japanese Laid-Open Patent Application No. H5-182631, a through-hole is formed so that an output opening has a larger bore diameter than an input opening, and thereby the inner surface of the through-hole has a tapered shape that becomes gradually wider toward the output opening. Therefore, a control electric field for guiding a secondary electron to the next stage enters the through-hole from the output opening larger in bore diameter, and rises along the inner surface on the side opposite to an inclined part, and deeply enters the inside of the through-hole. As a result, the strength of the control electric field that can enter the inside of the through-hole increases, and the emitted secondary electron can be more reliably guided to the next-stage dynode, thus making it possible to improve the gathering efficiency of electrons.